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Equatorial Platform for Dobsonian Telescopes

Moonshane

By Moonshane
in DIY Astronomer

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Moonshane

Moonshane

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I am starting this thread alongside the one by Sean http://stargazerslounge.com/diy-astronomer/125588-build-project-dob-equatorial-platform.html on a similar mission.

Still very much in the read everything you can online phase and changing my mind daily as to how it will be designed.

Broadly I will be following the design here Stellafane Links Page and specifically this one A large equatorial platform for a big Dob.

Doubtless there will be a number of changes as I go along. For now I am assembling parts and today's change was a rethink on how to power the motors, how to rewind the threaded rod which drives the platform, and how to prevent the platform moving too far.

I have come up with the attached wiring diagram which includes 4 RPM motor at one end of the rod (reduced with a dew controller to about 2.5 rpm) and a 60 rpm motor at the other end to 'return to base' in about 2 mins. At either end there's a limit switch that cuts off the power when the platform runner hits the switch. The on/off/on toggle switch will ensure the power is working the right motor at the right time.

I hope this makes sense and welcome any suggestions.

The dew controller will sit between the battery and the toggle switch on the positive side and between the motors and the battery on the negative side.

I'll post pics as I start work properly and also list the parts and sources as I go.

Cheers

Shane

wiring EQ motors.pdf

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Moonshane

Moonshane

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One thing I have just discussed with Sean and confused us both for a while is the relationship between centre of gravity of the scope and base, radius of the conic section for the north bearing and the dimensions of the EQP.

I have a COG of 35" on my 6" f11 dob and working like you, I got to a platform that was 5 feet long!! I now realise, as you have, that the it is the radius of the north sector in partic that is affected by the COG - i.e. that the higher the COG of the scope, the larger the radius of the conic section for the north sector (and thus the shallower the curve). I think this is reflected by your earlier comment that a six foot diameter sounded a bit large for my platform?

the only thing that has to affect the size of the platform boards is the physical size of the dob base/feet - i.e. it should fit on the thing! I also think that you can use a platform based on a higher COG than the scope you are using but not lower. In my case I have two dobs which have 35" and 25" COGs so as long as I base my dob on the former and they both fit on I can use it for both.

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Moonshane

Moonshane

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Hi all

this sounds off topic but I am planning to use some small DC motors that I got

from Ebay on my EQP. they are described as follows:

Specifications:

* Torque: 30 N*cm

* 12V DC

* 60RPM

and

Specifications:

* Torque: 250 N*cm

* 12V DC

* 4RPM

I also bought some micro switches and temporarily wired them up to 12v DC

tonight but they (the micro switches) blew when I closed the switch. I suspect

that they were rated at too low an amperage for the motors? I still want to run micro switches of some kind (or push to break switches) but need to know the amperage to ensure I don't keep blowing them and also more importantly so that I don't blow my dew controller that I hoped to use to control the speed to sidereal rate - this can handle 4A.

Is there a way to calculate the amperage from the above figures or would more

information be needed? Can anyone help please? Do I just need to buy or borrow an ammeter??

Best wishes and thanks

Shane

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SniffTheGlove

SniffTheGlove

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Hi all

this sounds off topic but I am planning to use some small DC motors that I got

from Ebay on my EQP. they are described as follows:

Specifications:

* Torque: 30 N*cm

* 12V DC

* 60RPM

and

Specifications:

* Torque: 250 N*cm

* 12V DC

* 4RPM

I also bought some micro switches and temporarily wired them up to 12v DC

tonight but they (the micro switches) blew when I closed the switch. I suspect

that they were rated at too low an amperage for the motors? I still want to run micro switches of some kind (or push to break switches) but need to know the amperage to ensure I don't keep blowing them and also more importantly so that I don't blow my dew controller that I hoped to use to control the speed to sidereal rate - this can handle 4A.

Is there a way to calculate the amperage from the above figures or would more

information be needed? Can anyone help please? Do I just need to buy or borrow an ammeter??

Best wishes and thanks

Shane

Can not work out the specs from the above details but this product from Maplins is rated up to 12A Low Cost Standard Microswitches : Micro Switches : Maplin and should be OK.

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Moonshane

Moonshane

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cheers Sean

I have sussed where I was going wrong - basically, I'm a muppet. now got the switches working as intended (had two left - only cost me £2 for the lot so only wasted £1!).

having discussed this with others I think that I need to run a 'flyback diode' across the terminals of the motor (the wrong way round so running pos to neg) of an appropriate amperage so that the current spikes created by connecting and disconnecting the motor don't wreck the dew controller.

I'll be putting a summary of all this on this thread when I get started - using blu tac connections at the minute!

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Deedub

Deedub

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Hi, this may be too late but...

In practice the current load for your motors will be changing depending on the changing load on them. It will also change with temperature.

But you could get a rough estimate if you can find the DC resistance of the motor winding/s from the specs, and use the simple

I = 12 / R

>Deedub<

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Moonshane

Moonshane

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cheers Deedub - my problem with this is that I cannot seem to get any info from the supplier. might try a spark mate who will probably have the correct meter. I also have just ordered an ammeter so can hopefully monitor it for a while and see how much it varies. as long as it's not too much I should be OK.

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Moonshane

Moonshane

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Well the work started in earnest on the EQP today (6/2/2011) with the bulk of the woodwork completed! The whole process started at about 9.30am today and ended about 2.30pm.

I had established that the radius for my north sector curve (the front section) was 27.5 inches and we used a piece of long M10 threaded rod to put the router far enough away from the pivot point which was a piece of steel angle screwed to a supporting board.

We then made the cut across the whole section and trimmed off the narrower end to ensure both were identical.

This created a narrow piece with a wide curve. We then set the saw blade on a bench saw at the appropriate angle (53 degrees) to leave a 'wedge shape of 37 degree on the curved section. This was not completely necessary but allowed more area for gluing and screwing / nailing the north segment to the bottom front of the top board. We made some small wedges to support the front edge of the sector.

Next we made short blocks which also had a 37 degree angle to act as the lower bearings on the ground board, front and back. These were again cut on the table saw and then chopped to length. I am using a pivot at the rear (south) rather than a curved section.

A key issue is getting the height of the centre of the pin at the rear to be the same as the bottom of the north segment. This ensures the EQP sits level at the middle of its run. We established that what appeared to be an ideal gam between the top of the ground board and the underside of the top board was 82mm. We found this 'leveling' was made much easier by the cutting of four blocks from scrap at 82mm each. This allowed the front and rear bearings to be set up and fixed in the correct position and ensured the whole things was level when at the mid point. We made to make a short piece at the rear to bring the groundboard bearing to the correct level.

This pretty much concluded the woodwork and the whole thing seems to be sitting correctly. We drilled a 10mm hole for the rear pin in the top and bottom bearings - more on this later. Another tip is to make a small template wedge with your latitude as one angle (53 degrees on my case) and this naturally gives you 37 degrees on the other side. It allows a quick check of all the angles before you cut and for setting saw blades etc.

We also cut some small 2.25" circles to act as feet - might use one or two depending on how strong they seem.

The whole thing stands at about 4.25" high without feet so not too bad really. I reckon I could shave off an inch or so.

I cut the ground board a little long (by 6") so that this gives me enough room for batteries and the motors and switches I plan to use.

There may be a little capacity to cut some of this off as required.

I will be painting this matt black to match the scope bases I have and will use this as a template for future further copies made to fit all dobs I own (assuming it works!).

I intend to fit the top surface of the top board with some sheet rubber to reduce any motor vibrations and the construction will include such considerations as I go. Some pics below and more to follow in due course.

Excellent progress so far!

Cheers

Shane

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Moonshane

Moonshane

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awesome. thx MS for taking the time out to take pics.

subscribed

thanks Lee

although the tools used were not something everyone has, I hope that showing the methods employed might encourage people that it can be done with very basic tools.

I'll be adding more pics as I make further progress. next job is sorting out a pin for the south bearing and making a teflon/washer bearing which I'll do tonight.

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LeoLion

LeoLion

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Have you considered a ball in a cone as your south bearing/pin Glide-Alwayse Ball Transfer Units and Fixing Sockets . The cone could be on either the mobile or the static part .

A Dob Platform is on my sometime list ( but I need to complete the cloud remover first :) . I'll keep watching (and wish you luck ).

I would think CPC or RS do the ball devices as a standard item.

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SniffTheGlove

SniffTheGlove

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You have done well Moonshane, I have only marked out the segment so far and today is hopefully be a good one if the morphine kicks, also got the slab man coming this morning for 3 days to lay my observing patio outside my Obsy so while he is at it I might as well go in the garage and start practising some routering.

PS, I like the old woodworking bench in the first few photos, just like my dads.

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Moonshane

Moonshane

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Hit some problems tonight.

I made a teflon bearing by sandwiching a piece of 3mm teflon between two 50mm washers and then trimming with a knife. I then cut the head off a M10 bolt and got it to fit the depth of the holes drilled in either side of the south bearing surfaces. My idea was that I would glue the washers to the wooden bearing surface and also glue the post into the bottom section of the south bearing. I thought this would create a stable fixing for the unit and it probably will. Unfortunately though when I put this together for a test it does not seem to be correct.

I have drawn a line with the north bearing at the mid point and then at either side of the base board. Anyone know what's up? Have double checked all my angles. I expected the lines to all revolve around the same radius? It seems the circular section won't run on the two bearings?

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